The invention is directed to the field of microsensor mass air flow devices and a method of fabricating the devices. The structure at an intermediate point of fabrication is a single crystal silicon substrate chip having on a portion of the surface a thin sacrificial layer of nickel-iron, titanium-tungsten, chrome, aluminum or other selectively etchable layer which delineates the future location of a web or diaphragm to be formed at a later step. The sacrificial layer and the rest of the silicon chip surface are coated with a thin film of silicon nitride which forms the diaphragm. By sacrificial layer is meant a layer which is removed at a later stage in the fabrication process. In this invention the pit or cavity in the silicon beneath the diaphragm is formed by backside etching. In this invention a precisely defined web or diaphragm area can be fabricated by backside etching without regard to dimensional variations of the backside etch pattern or tolerance variations in the thickness of the silicon wafer and without the necessity of accurate alignment of backside features with front side features. It is not necessary to dope the silicon to achieve the desired precision of position and area of the web.
In the fabrication of a thin film web on the surface of a silicon wafer for the emplacement of a sensing thin film material such as in an airflow sensor using heated resistors, it is necessary to accurately reproduce the area, shape, and position of the web. In the simple backside etching technique of prior art it is quite difficult to accomplish this by the usual photolithographic techniques because of difficulties in aligning front features with back features on the opaque wafer, and because wafer thickness variations directly affect the area defined for the web on the front surface.
One prior art improvement on this situation as shown in FIG. 1 is to heavily dope with boron an area that accurately defines the web and allow the backside etch pit apex to fall on the boron etch-stop area comfortably outside the defined web area. However, the thickness of the doped layer can only be a few microns and may not supply the desired rigidity in the web support, and may not supply as much thermal heat sinking as would be desired at the edge of the web if thick silicon were present.